1. Field of the Invention
The present invention is generally directed to a depth sounder, such as employed in fish finding apparatus. More particularly, the present invention is directed to a depth sounder and method for eliminating noise.
2. Description of the Related Art
The use of sonar echo sounding for recreational and commercial fish finding purposes, and for other underwater object identification, is widespread. These devices utilize sonar signals to develop a view of underwater environment.
One major problem faced by manufacturers of depth sounders is that, underwater environments are often noisy and the noise levels can, and do, vary widely. This noise interferes with accurate underwater detection and makes it difficult, both for the device and the user, to interpret an underwater environment.
In order to alleviate the problem of noise, many prior depth sounder products utilize what is commonly known as a detection threshold, of a selected amplitude, such that sonar echo signals, which correspond to sonar pulses transmitted from the sounder device and which have reflected off of the bottom of a body of water or off of any object in the water, that are louder than the detection threshold are accepted, while sonar echo signals that are quieter than the detection threshold are rejected. More particularly, such products typically display, on a display screen, data corresponding to sonar echo signals which are louder than the detection threshold, but do not display data corresponding to sonar echo signals which are quieter than the detection threshold.
As will be appreciated, the placement of the detection threshold is a critical step in the manufacture of such depth sounders. In this regard, the higher the detection threshold value, the more noise that will be rejected and the more likely echo signals received by the depth sounder will be accurately interpreted as underwater environment. However, a high detection threshold has the drawback of potentially rejecting weak signals that, when compared with a lower detection threshold, would have been received and interpreted. Conversely, a low detection threshold allows weaker (and thus a potentially greater number of sonar echos) to be received and interpreted, but increases the likelihood that noise will be incorrectly interpreted as underwater environment.
While there are many theories and practices relating to the proper placement of the detection threshold, the need remains for a sonar depth sounder which efficiently adjusts the detection threshold based upon a change in noise levels. The present invention fills this need and other needs, in a unique manner.
A sonar depth sounder of the present invention has a processor. An input, a display, and a memory, are connected to the processor. The processor is connected to a transmitter/receiver, which is in turn connected to a transducer. In use, the transmitter transmits a plurality of signals, which are emitted from the transducer as sonar signals towards the bottom surface of a body of water. The receiver receives sonar signals reflected back from the bottom surface of the body of water, and reflected from any objects resting on the bottom surface of the body of water or suspended between the top surface of the body of water and the bottom surface of the body of water.
In accordance with an aspect of the present invention, the processor first takes a passive noise interrogation such that the receiver receives sonar signals that are not echo signals received in response to a corresponding transmission from the transmitter. Rather, the processor receives, from the receiver, a signal representative of the ambient noise in the underwater environment. The processor processes that signal to calculate a detection threshold value, and stores the detection threshold value in the memory of the sounder device.
More particularly, the processor calculates a mean of the sonar signal representative of the environmental noise in the underwater environment, and also calculates a variance of the signal. Specifically, the processor determines the detection threshold (DT) value according to the following equation:
DT=mean+xcex1variance
where mean is the mean of the signal indicative of the ambient noise in the underwater environment, variance is the variance of the signal indicative of the ambient noise in the underwater environment, and a is a scaling factor. The scaling factor a is determined through testing, and is preferably approximately 7. Other scaling factors, however, could be utilized.
Once the detection threshold value is stored in memory, the processor causes the transmitter to transmit a plurality of signals, which are emitted from the transducer as sonar signals, towards the bottom surface of the body of water. As stated, the receiver receives reflected echo sonar signals back from the bottom surface of the body of water, and reflected from any objects on the bottom surface or suspended between the top and bottom surface of the body of water. The processor receives electrical signals indicative of the reflected sonar signals, and determines whether these signals are of an amplitude greater than the detection threshold. When the signals are greater in amplitude than the detection threshold value, the processor displays data, on the display, indicative of the underwater environment. For example, in accordance with known techniques, in the event a reflected echo signal is believed to indicate that a fish is located within the body of water, an icon indicative of a fish is displayed on the display.
In accordance with an additional aspect of the invention, from a point in time at which the transmitter transmits a sonar signal into the body of water, the detection threshold value is increased over time to compensate for increased gain associated with the receiver. In this regard, it will be understood that for sonar depth sounders which an increase of gain over time, the detection threshold value ramps upwardly linearly over time at a slope that is determined through testing. It will be understood, however, that increasing the detection threshold value over time could be accomplished in other manners.
In an alternate embodiment of the present invention, data indicative of ambient noise in an underwater environment, such as ambient noise from a boat motor or water moving past a transducer of a sonar depth sounder, is monitored and eliminated in accordance with a time-varying detection threshold methodology. In particular, a preliminary value corresponding to actual or predicted ambient noise is determined. As discussed, this preliminary value may be determined by taking a passive interrogation (e.g., listening to noise in an underwater environment that is not in response to an echo pulse from the sonar depth sounder). In this way, the ambient noise within the underwater environment to be interrogated is received by the sonar depth sounder, and converted into a data value indicative of the underwater ambient noise. Alternatively, this preliminary value may be established according to a user input. For example, the sonar depth sounder of the present invention may include a knob for varying the level of data rejection desired and, for example, may have corresponding inputs associated with xe2x80x9clowxe2x80x9d, xe2x80x9cmediumxe2x80x9d, and xe2x80x9chighxe2x80x9d ambient noise rejection levels. In this case, corresponding low, medium, and high data values are stored in the memory in association with the respective low, medium, and high inputs, such that upon activation of one of the inputs, the corresponding data is recalled and used as the preliminary numeric value intended to be indicative of ambient noise conditions in the underwater environment.
Once the preliminary value corresponding to a rejection level is established, the sonar depth sounder processes an active interrogation cycle, in which a sonar pulse is transmitted into the underwater environment. As described, and as will be understood, corresponding echo pulses are received by the sonar depth sounder. In accordance with an aspect of the invention, data indicative of the received echo pulses are digitized and stored in a memory. In accordance with a further aspect of the invention, a continuous-time averaging technique is applied to the sequence of data corresponding to the interrogation cycle. This averaging process is preferably carried out by applying a low pass filtering technique to the data sequence, although other continuous-time averaging techniques, such as correlative window techniques may be employed. The result of the continuous-time averaging process is an averaged (e.g., filtered) value for each sample of the data. This resulting sequence of averaged (or filtered) data provides a time varying detection threshold.
Upon completion of the continuous-time (e.g., filtering) process, lower and upper limits are applied to the detection threshold. In other words, any filtered sample having a value more than the lower limit, or higher than the upper limit, is clipped to the corresponding limit. A numeric measure associated with these limits is based upon the preliminary established value (e.g., that value associated with the actual or predicted ambient noise conditions of the underwater environment).
Following application of lower and upper limits to each sample of the averaged filtered data, a sample by sample comparison is made in which each sample of the filtered (and possibly clipped) data is compared with a corresponding sample of the data originally received. During this comparison process, when a value associated with an actual sample is less than the filtered sample, thus meaning that the actual sample is beneath the detection threshold, the processor of the depth sounder of the present invention rejects the sample, and does not display data corresponding to the rejected sample. When, however, the actual sample has a data value that is greater than the data value of the filtered sample, processing advances to the next sample for performing the necessary comparison.
In accordance with a further aspect of the invention, any data above the detection threshold, but that is shorter in duration than a lower time limit, is rejected. Accordingly, data will only be displayed on the display of the sonar depth sounder when each of a series of consecutive samples is greater than the corresponding filtered samples, and wherein the time frame associated with the series of consecutive samples is greater than the lower time limit. Thus, data indicative of a spike, which clearly surpasses the detection threshold, but which is shorter in duration, will be eliminated. Additionally, the time limit utilized in making the determination of whether data should or should not be displayed on the display is based upon the preliminary established rejection value. In other words, in the case where the preliminary established value is based upon ambient noise conditions, when the ambient noise conditions in the underwater environment are relatively low, the corresponding time limit that data believed to be indicative of underwater noise must remain above the detection threshold is correspondingly low. However, when the ambient noise has been determined to be relatively high, the time limit utilized for rejecting data is longer, such that a greater number of consecutive data samples must remain above the detection threshold in order for the data to be displayed.